HP DC5000 will not boot with old Hard Drive

you can not take a boot hard drive from an existing computer and use it on a new computer as a system boot drive

I do that all the time just fine.
I also make image of old drives and image it to new PC drive just fine also.

This is the first time I deal with a HP and also the first time I get stuck!

Usually the boot hard disk that is preloaded with windows will only work with the existing motherboard it came with as a proprietary measure to make sure you are buying a new copy of windows for each system. You would need a new copy for your new build in order to reuse the drive.

Note: I've tried to fix widows XP with the XP from the old hard drive and it fixed with no problems (no error) but when it reboot it goes to "blank screen"

what you can try doing is loading the old drive back into the old computer along with a new drive than using image software to create an image of the drive... sometimes it will work sometimes it won't, but it's very rare that you can use a preloaded windows version on a computer of a different model with a different motherboard.

PS: I don't want to just use the drive (I can do that by placing it as a slave and it works fine) I need to boot with it. (It has all my software installed on it and it would take me a week to re-install everything)

I want to use my old drive as boot-able and place the "new" drive as a slave

Thanks for helping... (I know I should have stayed away from HP computer)

again what I am trying to tell you is most computers with a preloaded version of windows will NOT let you move them to another computer. One way around that is to slave anther drive and copy an image of your working drive to the old one making a new drive with the same software on it, but none of the built in record that was designated by the original builder. HP is notorious for making their computers very non-upgrade friendly so this is probably your best bet for reusing the data and copy of windows on the drive you have.